Recent signals from Brussels have buoyed hopes that the United Kingdom and the EU can agree and approve a proper divorce agreement before the UK leaves on March 29, though the sides are still divided on about one fifth of the detail of a deal.

Ralf Speth also said the company would not be able to build cars if post-Brexit customs checks meant that the motorway to and from the southern English port of Dover, which is used to transport components, becomes a "car park" due to snarl-ups.

Juncker told German broadcaster ZDF on Friday that the EU would not let anyone determine its trade policies. If Washington decided to imposed tariffs on vehicles after all, he said, "then we will also do that".

The proposal says it would avoid checks on the border between Northern Ireland and Ireland and protect manufacturing supply lines, while fulfilling domestic promises to end the jurisdiction of the European Court of Justice (ECJ), control migration and allow Britain to establish its own trade policy.